Are you referring to the short section of the pin slot being filled with casting material? I suspect that the extra metal in the slot is so the builder can custom fit the wedges to his stock, cutting away the material as required to make the wedges just clear the barrel tenons as they are withdrawn to remove the barrel. When I made a TRS Baker for a local fellow, I don't remember wondering about the extra metal at the time. I likely just used a hack saw and files to remove it.
